Viral hepatitis B is a widely-spread, extremely harmful disease. It has been reported that approximately 200,000,000 people are chronically infected with hepatitis B virus (HBV), and numerous deaths result from it and from conditions associated with it.
In the sera of some HBV infected patients, there are 42 nm infectious virus particles comprising viral surface antigen (HBsAg) which is the proteinaceous coating of the viral particle, viral core antigen (HBcAg) and viral DNA. In addition to the whole virus particle, the blood of infected individuals contains smaller (20 to 22 nm) spherical and tubular forms that are made up of HBsAg. The HBsAg particles can induce specific antibodies and bestow immunity against infection. However, isolation of the particles from the blood is too expensive, and supplies are too limited, to permit their successful use in mass vaccination in areas where HBV is highly endemic. Besides, there might occur potential hazards due to undiscovered pathogens. Accordingly, alternative methods of producing HBV surface antigens are actively being sought.
HBsAg consists of three envelope proteins of different sizes. They are encoded in the same reading frame by the HBsAg gene, but starting from three different translation initiator ATG codons. The three proteins are the S protein (or major protein with 226 amino acids, encoded by the S region of the HBsAg gene; the minor middle surface protein (MS protein) which contains an extra 55 amino acids attached to the N-terminus of S-protein, i.e. having a total of 281 amino acids, the extra 55 amino acids being encoded by the preS2 region of the gene and containing the receptor for polymerized human serum albumin (pHSA); and minor large surface (LS) protein encoded by all three preS1, preS2 and S regions of the gene, and consisting of about 400 amino acids. MS protein and LS protein are found mainly on the envelope of the viral particle, and in HBsAg particles of sera from infectious patients.
Since HBsAg particles can induce specific antibodies and bestow immunity against infection, their biology and application to vaccine preparation have attracted wide attention. For the most part research was focused on the S-protein, but more recently on the MS and LS proteins as well. Since the MS and LS contain additional epitopes not present on the S protein, their inclusion in synthetic or recombinant vaccines has been suggested (Milich, D. R., G. B. Thornton, A. R. Neurath, S. B. Kent, M. L. Michel, P. Tiollais and F. V. Chisari (1985) Science 228. 1195-1199).